<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<input type="text" id="text_a" />
		
		<select name="chose" id="s_chose">
			<option value="a">+</option>
			<option value="b">-</option>
			<option value="c">*</option>
			<option value="d">/</option>
		</select>
		
		<input type="text" id="text_b"/>
		
		结果:<input type="text" id="text_res"/>
		<button type="button" id="calc">计算</button>
	</body>
	<script type="text/javascript">
		document.getElementById('calc').onclick=function(){
			var chose=document.getElementById('s_chose').value;
			var text_a=parseFloat(document.getElementById('text_a').value);
			var text_b=parseFloat(document.getElementById('text_b').value);
			if (chose=="a") {
				document.getElementById('text_res').value= (text_a+text_b).toFixed(4);
			} else if(chose=="b"){
				document.getElementById('text_res').value= (text_a-text_b).toFixed(4);
			}else if(chose=="c"){
				document.getElementById('text_res').value= (text_a*text_b).toFixed(4);
			}else if(chose=="d"){
				document.getElementById('text_res').value= (text_a/text_b).toFixed(4);
			}
			
		}
		
	</script>
</html>